This invention relates to a process of synthesizing crystalline porous solids. More specifically, this invention relates to the synthesis of crystalline porous silicas and metallosilicates, such as silica molecular sieves, aluminosilicate zeolites, and for the purposes of this invention, clathrasils.
In other aspects, this invention pertains to novel compositions of zeolite P1, zeolite beta, and sodalite.
Crystalline porous silicas and metallosilicates find utility as supports for catalytic metals. Sodalite, for example, is useful as a catalyst support. Crystalline porous metallosilicates also have utility as heterogeneous catalysts in organic processes. For example, zeolite beta is useful as a catalyst in alkylating and transalkylating aromatic compounds and also useful as a catalyst in cracking, hydrocracking, and dewaxing hydrocarbons. Crystalline microporous solids are also employed as ion-exchangers and as sorbents for purifying gases. Zeolite beta, for example, is used as a sorbent for C.sub.8 and C.sub.10 alkylbenzene separations. Zeolite P1 is used in the separation of linear and branched hydrocarbons.
Crystalline porous solids are commonly prepared by hydrothermal methods. A typical preparation involves heating one or more nutrients, such as a source of silica, and optionally a source of a metal oxide, such as alumina, in water in the presence of a mineralizer and a structure directing agent until crystallization occurs. The mineralizer, which is typically hydroxide or fluoride, functions as a solubilizer of silica and metal oxides transporting them through the reactive solution or gel to nucleation sites. The structure-directing agent includes templates and stabilizers. The template, which may be a cation or neutral species, tends to favor the nucleation and growth of a particular zeolite. The stabilizer, often referred to as a pore filler, functions in a stabilizing role and may be required for a successful synthesis. Water and organic bases, such as primary, secondary, and tertiary aliphatic amines, and tetraalkylammonium halides are common stabilizers. Additionally, the template or stabilizer may assist in controlling pH and/or may provide charge balance with a counteranion or the anionic framework.
It is known that crystalline porous solids can be synthesized in water to which dilute aqueous or liquid ammonia has been added. U.S. Pat. Nos. 4,452,907 and 4,452,908, for example, disclose processes for the preparation of crystalline aluminosilicates, such as MFI zeolites, involving mixing a source of silica, a source of alumina, a source of an alkali metal (MOH), water, and a source of ammonium ions, such as ammonia or an ammonium salt, in the absence of alcohol or alkylene oxide, and maintaining the reaction at elevated temperature for a period such that crystallization occurs. The molar composition of the reaction mixture is taught as follows:
SiO.sub.2 :Al.sub.2 O.sub.3 greater than 12:1 PA1 MOH:Al.sub.2 O.sub.3 from 1:1 to 20:1 PA1 SiO.sub.2 :NH.sub.3 from 1:1 to 200:1 PA1 H.sub.2 O:MOH from 30:1 to 300:1 PA1 (a) ammonia, PA1 (b) water in an amount such that the molar ratio of ammonia to water is between about 4 and about 26, PA1 (c) one or more nutrients comprising a (hydrocarbyl)ammonium polysilicate hydrate salt, and optionally, a source of a metal oxide which is reactive in the reaction mixture, and optionally, a source of an additional cation which is reactive in the reaction mixture, the nutrients being present in amounts sufficient to prepare the crystalline porous solid, and PA1 (d) a mineralizer in an amount sufficient to mineralize the nutrients,
Based on the above and as shown in the examples, the reaction mixture is predominantly aqueous.
U.S. Pat. No. 4,717,560 discloses a hydrothermal process for preparing zeolite ECR-5 of the cancrinite structure involving forming a mixture containing aqueous ammonia, a source of silica, a source of alumina, and sodium hydroxide, and aging the reaction gel until crystallization occurs. It is taught that aqueous ammonia is present in the reaction mixture generally in an amount from about 15 to about 50 mole percent, based on total moles of water, and preferably, in an amount from 20 to 30 mole percent. It is further taught that "[t]he ECR-5 product may be formed in reaction mixtures containing more than 30 mole percent ammonia. However, such high ammonia controls typically mandate pressurization of the reactor."
D. E. W. Vaughan and K. G. Strohmaier report in the Proceedings of the 9th International Zeolite Conference, Montreal, 1992, Eds. R. von Ballmoos et al., 1993, Butterworth-Heinemann, that zeolites can be synthesized in aqueous ammonia solvents. Cancrinite products are reported up to an ammonia/water molar ratio of 3; however, no crystalline products are recovered from liquid ammonia, because the solubilities of the reagents are too low. The examples typically employ an ammonia/water ratio of 0.4.
In hydrothermal syntheses the composition and structure of the products obtained are limited to those products whose precursor polysilicates and polymetallates are readily stabilized by water solvation. It would be desirable to have an alternative reaction environment for growing crystalline porous solids which stabilizes novel precursor polysilicates and polymetallates such that crystalline porous solids of novel composition and structure are formed. It would also be desirable to employ in the syntheses of crystalline porous solids novel reagents which form reactive intermediate species.
In addition to the above, it would be advantageous to synthesize crystalline porous solids directly in an ammonium form, rather than a metal ion, e.g. sodium, form. The advantage relies on the fact that usually the catalytically active form of these solids is the acid form. When the solid is synthesized as a metal ion salt, it must be ion-exchanged with aqueous acid solution to obtain the acid form. When, however, the solid is synthesized in the ammonium form, it need only be heated to drive off ammonia to obtain the acid form.
Even though the advantages of alternative reaction environments are recognized, the prior art does not teach the synthesis of crystalline porous solids in concentrated ammonia. Instead, as shown in U.S. Pat. No. 4,717,560 and in the reference of Vaughan and Strohmaier, op. cit., the disadvantages of working in ammonia are set forth. Chief among these disadvantages is the low solubility of common sources of reactive silica.
In other aspects, R. Szostak reports in Molecular Sieves: Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989, p. 64, that zeolite Na-P1 is known with silica/alumina molar ratios between 2 and 8. U. H.ang.kansson and L. F alth report in Acta. Cryst., C46, 1363-1364 (1990), the synthesis of zeolite Na-P1 having a Si/Al atomic ratio of 3.47 (SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 6.94). The crystalline structure of Na-P1 is further discussed by Ch. Baerlocher and W. M. Meier, in Zeitschrift f ur Kristallographie, Bd. 135, S. 339-354 (1972). Disadvantageously, the composition of zeolite P1 is limited to low SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios from 2 to about 8.
R. Szostak also reports in Molecular Sieves: Principles of Synthesis and Identification, op. cit., p. 64, that zeolite beta is known with SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios between 5 and 100. U.S. Pat. No. 4,840,929 discloses zeolite beta having SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios up to 1500. The crystalline structure of Na-beta is presented by J. M. Newsam et al. in Proceedings of the Royal Society of London, A 420, 375-405 (1988). These references are silent with regard to an all-silica beta composition. The synthesis of zeolite beta by traditional hydrothermal methods is also known as disclosed in U.S. Pat. Nos. 4,923,690 and 5,164,170. Disadvantageously, one method produces only partially crystallized beta, while the other method requires a seed crystal. Use of seed crystals is not preferred.
In yet another aspect, zeolite sodalite can be prepared by hydrothermal methods, as taught in the following references: R. M. Barrer and P. J. Denny, Journal of the Chemical Society, 971-982 and 983-1000 (1961); and R. M. Barrer and D. E. Mainwaring, Journal of the Chemical Society, Dalton, 2534-2546 (1972). Normal sodalite crystallizes with a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 2; however, a silica-rich sodalite is known having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 9.5, as reported by R. Aiello and R. M. Barrer in the Journal of the Chemical Society (A), 1470-1475 (1970). Disadvantageously, the silica/alumina molar ratio is limited to values of between 2 and about 10.